Brain Case: Diplodocus longus

In a corner of the exhibition The World’s Largest Dinosaurs, an elegant wire outline of the head of Diplodocus longus, a sauropod that lived in the Late Jurassic period about 156 million years ago, anchors a fascinating fossil: one half of a bony braincase, its interior carefully color-coded to denote various functional structures once within it.

One’s ﬁrst impression is how very small the brain must have been, especially given that the brain itself probably took up about only 70 percent of the bony case, with protective outer layers called meninges taking up the other 30 percent. Despite the dinosaur’s massive size—it was 80 feet long and weighed 20,000 pounds—its brain weighed about 4 ounces. By comparison, the average adult human brain weighs 48 ounces.

Of course, scientists can only speculate about this because brains—composed mostly of water—don’t fossilize well and are extremely rare, almost nonexistent, within the fossil record.

“It’s informed guesswork,” says Jonah Choiniere, a postdoctoral fellow in the Division of Paleontology. “We’re extrapolating based on comparative data from living animals such as birds and crocodiles.”

Though the brain itself is lost forever, the endocast, or cavity within the braincase, offers valuable clues to the dinosaur’s basic functions, metabolism, and lifestyle. In the exhibition display, four key structures are identiﬁed with different colors: the optical nerve opening (green); the facial nerve opening (orange); the pineal opening (yellow); and the site of the pituitary gland (blue).

“This particular specimen was chosen for this exhibition because it has been delicately prepared and sectioned to expose the brain cavity,” says Mark A. Norell, chair of the Division of Paleontology and curator of The World’s Largest Dinosaurs. “Only rarely are sauropod dinosaur skulls found with the braincase relatively intact and undistorted.”

Color-coding the braincase for the current special exhibition involved a process designed to be fully reversible. First, preparators covered the interior with a layer of acetone-soluble plastic—a substance commonly used to harden fossils. Then, they applied acrylic paint, which can be peeled off the plastic base that in turn can be removed with acetone, a solvent familiar to anyone who has ever used nail polish remover.